A known cellular telephone system is a Code Division Multiple Access (CDMA) system. The CDMA wireless phone system allows multiple cellular phone users to share the same frequency spectrum, and uses a generated noise carrier with a different and essentially orthogonal instance of the noise carrier assigned to each mobile unit within a cell. The base station receiver in a CDMA station correlates the received signal from a mobile unit with the desired noise carrier, extracting the transmitted digital signal with a sufficient signal-to-noise ratio to achieve a satisfactory data error rate. Because the base stations in a system such as CDMA must be synchronized with surrounding base stations to handle handoff of mobile phones between cells and for other functions, a time reference must be provided to each base station. This is commonly provided via Global Positioning System (GPS) receivers which comprise a part of each base station.
GPS satellites each provide radio signals that are synchronized and usable by GPS receivers not only to derive one's physical position relative to the satellites but also to derive a very accurate time reference. Because the GPS receiver antennas of cellular phone equipment are often placed high relative to surrounding terrain, they are subject to lightning damage in addition to physical damage from rough handling or other damage. CDMA base stations which lose contact with GPS satellites should ideally continue to operate during this holdover period until contact can be reestablished, whether through repair of damaged equipment, or other changed circumstances. A crystal oscillator may provide a time reference during this holdover period, as long as the oscillator is stable enough to keep the base station sufficiently synchronized with other base stations.
A method is needed for improving on the performance of current CDMA base station clock stability when the base station is not receiving a GPS signal to provide a clock reference. When no GPS signal is received the system clock operates in holdover mode, and the clock signal is generated by a crystal oscillator designed to provide a signal of the same frequency as is provided by the GPS receiver. It is necessary to improve current GPS-based clock stability during holdover by compensating for the performance of a crystal used to generate a clock signal during this holdover period.
A typical CDMA base station uses a received GPS signal to produce a reference clock signal to ensure that CDMA stations are synchronized in operation. Such synchronization between stations is important to the proper operation of a CDMA system, as common operations such as a CDMA spread spectrum code search and station-to-station handoff require that stations be closely synchronized in time. Mobile stations also synchronize to the signals provided by the base station, such that the GPS clock provides a timing reference for both the base station and all mobile stations active within the cell.
This synchronization is jeopardized when the base station fails to receive a GPS signal, and must rely on an oscillator to maintain time independent of the GPS signal still used by neighboring base stations. This commonly occurs as a result of lightning strikes that damage the GPS antenna or receiver of a CDMA system, and also occurs as a result of damage due to rough handling and vandalism or from other causes. If the oscillator is not sufficiently stable, the time it provides to the base station may drift with respect to the desired GPS reference time, and cause the base station to fail to communicate properly.
Currently, a new oven-controlled crystal oscillator (OCXO) used to provide a holdover clock signal in a CDMA system is burned in and tested in operation for frequency stability for no more than a few days. Crystals that perform adequately are then accepted for service and placed in use as part of a CDMA base station. But, because crystals often take from 20 to 30 days to settle in or become stable in performance, this test cannot ensure performance of the crystal in extended operation.
Excessive frequency drift due to molecular settling or spurious frequency jumps due to contaminants in the crystal may cause the crystal to perform much more poorly in the field than these preliminary tests could indicate. Other factors such as rough handling during installation or spurious mechanical changes in the crystal may further degrade crystal stability, and are not detectable after initial testing. In order to reduce base station cost, lower cost reference oscillator may be used. However, in general, the frequency stability of low cost OCXOs is poor and thus, the frequency stability in the holdover period is not reliable. It must be ensured that the increased frequency drift in the low cost OCXOs used as base station reference oscillators is compensated sufficiently to maintain the required level of base station synchronization.
A clock synchronizing circuit including a phase lock circuit with a voltage controlled oscillator (VCO) and a temperature compensation circuit is disclosed in European Patent Publication No. 0 615 360 A2 (Sep. 14, 1994). A frequency standard generator including a voltage controlled crystal oscillator (VCXO) that is phase locked to a GPS satellite time signal is disclosed in U.S. Pat. No. 6,081,163 issued to Ujiie et al. on Jun. 27, 2000. Bruce M. Penrod “Adaptive Temperature Compensation of GPS Disciplined Quartz and Rubidium Oscillators”, 1996 IEEE International Frequency Control Symposium, pp. 980-987, discloses that an algorithm for performing adaptive temperature and aging compensation of GPS disciplined oscillators.
U.S. Pat. No. 6,194,970 issued to Nielsen et al. on Feb. 27, 2001 discloses an oscillator stability monitoring and compensation system for analyzing the steering voltage applied to a crystal oscillator over time and compensating for spurious frequency jumps in determining the drift rate of a crystal oscillator. The steering voltage is used to estimate oscillator stability by comparing a projected steering voltage against an actual voltage after a simulated holdover period, or analyzing a steering voltage recorded over a period of time and evaluating rates of change. Spurious frequency jumps are removed from data collected while not in an actual holdover, making the data more accurately represent the frequency drift rate of the oscillator. The rate of occurrence of spurious frequency jumps while not in holdover may be monitored to provide information regarding the physical condition of the crystal. However, the system is directed to detect spurious frequency jumps in the crystal and compensate for these jumps in character of the performance of the crystal. It does not address the problem of a low performance reference oscillator.
U.S. Pat. No. 5,697,082 issued to Greer et al. on Dec. 9, 1997 discloses a self-calibrating frequency standard system self-calibrating a clock of a communication terminal for use with communication systems in which a central communication node generates time base correction signals for the terminal clock. Included is a terminal oscillator which generates an oscillator frequency that includes an error amount. An oscillator calibration filter generates a frequency error estimate amount. The frequency error estimate amount generated by the calibration filter is subtracted from the oscillator frequency error amount. The time base correction signals are applied to the calibration filter to thereby modify the frequency error estimate amount generated by the calibration filter based upon the time base correction signals generated by the communication central node. It does not address the problem of low performance reference oscillator, either.
For example, in the North American synchronous CDMA, it is required to meet with all reference oscillator specifications with respect to the accumulated timing error over the holdover period. An example of specification for a compact base transceiver system (BTS) is 6.9 microseconds cumulative timing error in 24 hours, which translates to a frequency stability requirement of 0.08 parts per billion on the 10 MHz reference oscillator. It is noted that the holdover specifications include all operating condition variations, the most severe of which from a crystal oscillator design standpoint, is the −5 Celsius to +70 Celsius ambient temperature range.
The simplest approach to the problem of determining the oscillator frequency when a more stable reference is unavailable is to avoid the requirement for doing so altogether. In order not to have to know the oscillator frequency during the holdover period, the problem is that one of ensuring the cumulative time error of the free running oscillator remains within the specified 6.9 microseconds. To achieve the required level of stability directly from the oscillator without recourse to any form of control loop places severe demands on the crystal cut and thermal stabilization through the use of double oven architectures. It is, thus, required to improve the oscillator long term stability with low cost. For example, rubidium oscillators and conventional double oven crystal oscillators are still expensive.